1. Field of the Invention
The present invention relates to a process input/output system, and more particularly to a process input/output system which is preferably used in a sequence controller or sequencer for controlling input and/or output in accordance with programs by a microcomputer.
2. Description of the Prior Art
FIG. 13 shows an example of the general arrangement of a prior art sequencer. In FIG. 13, reference numeral 1 denotes a central processing unit (CPU) in the form of a microprocessor. Reference numerals 2S and 2M denote memory portions, for storing sequence control command programs and system programs respectively. Reference numeral 3 denotes a memory portion having a random access memory (RAM). Reference numeral 4 denotes an input portion, reference numeral 5 denotes an output portion and reference numeral 6 denotes a bus.
Known circuits having discrete components and logic ICs may be used as the input portion 4 and the output portion 5 in this type of sequencer. The number of such circuits correspond to the number of inputs and outputs.
FIG. 14 and FIG. 15 show an example of an arrangement of such a conventional input portion and a conventional output portion, respectively. In these drawings, reference numerals 11 denote a terminals to be connected to an external apparatus, reference numerals 12a-12h denote resistors, reference numeral 13 denotes a capacitor, reference numeral 14 denotes a buffer, reference numeral 15 denotes a photo-coupler for insulating the sequencer electrically from the external apparatus, reference numeral 16 denotes a latch register, reference numeral 17 denotes a bus driver and reference numeral 18 denotes a transistor. Moreover, reference numerals B0-B7 denote bus signal lines, and *RD, *WT, *CS and *TED denote a data read signal, a data write signal, a bus selector signal and a data store signal, respectively, from the control portion 1.
This type of process input/output system for a sequencer has the disadvantage, however, that the number of components given by (the number of components per one input or output) X (the total number of inputs and outputs) is needed. Accordingly, if the number of inputs and/or outputs is increased, the volume of the system expands and the cost increases. Moreover, there is the disadvantage that it is not easy to determine circuit constants when designing a sequence controller.